The mechanism of anaphylactic reaction will be studied in different species using different classes of homologous and heterologous antibodies. Antibodies mediating sensitization will be studied in different species, especially in guinea pigs and mice. The two classes of homologous antibodies which mediate immediate hypersensitivity, namely, the gamma 1 and the reaginic gamma E-like antibody will be investigated. In vitro studies using mouse myeloma proteins will be made using mouse mastocytomas, and the receptors on mouse mast cells and mouse mastocytomas with homologous and heterologous mouse antibodies will be investigated using the indirect rosette technique.